staging: vt6656: Power save stop wake_up_count wrap around.

conf.listen_interval can sometimes be zero causing wake_up_count
to wrap around up to many beacons too late causing
CTRL-EVENT-BEACON-LOSS as in.

wpa_supplicant[795]: message repeated 45 times: [..CTRL-EVENT-BEACON-LOSS ]

Fixes: 43c93d9b ("staging: vt6656: implement power saving code.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/fce47bb5-7ca6-7671-5094-5c6107302f2b@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6656/usbpipe.c

@@ -207,7 +207,8 @@ static void vnt_int_process_data(struct vnt_private *priv)
 				priv->wake_up_count =
 					priv->hw->conf.listen_interval;
 
-			--priv->wake_up_count;
+			if (priv->wake_up_count)
+				--priv->wake_up_count;
 
 			/* Turn on wake up to listen next beacon */
 			if (priv->wake_up_count == 1)